Cosmogenic nuclides

Cosmogenic nuclides are unstable isotopes that are produced by interaction of cosmic rays with the nucleus of an atom in the Earth’s upper atmosphere. Some are incorporated into biological systems (e.g.14C) or are carried down to the solid surface of the Earth (e.g. 10Be, 26Al) where their abundances can be measured to date biological or physical processes.

Cosmogenic nuclide production in Earth’s upper atmosphere occurs mainly by interaction of atomic nuclei in gases or dust in the atmosphere with energetic cosmic rays, mainly protons but also with neutrons generated by such interactions.

The radioactive isotopes of the elements beryllium, carbon, aluminium, chlorine, calcium, and iodine are usually measured by Accelerator Mass Spectrometry. Their lifetimes range from thousands to millions of years.

The table above gives the half life (time it takes for half of the atoms to decay; see here) and the detection limit (value for the atom ratio of the radionuclide to stable nuclide measured for a “blank” sample that contains a negligible amount of the radionuclide).

Decay products of some of these radioactive nuclides (10Be, 26Al, 129I) have also been detected in meteorites and other extraterrestrial materials.

Cosmogenic nuclides typically occur in very low abundance and are measured using Accelerator Mass Spectrometry.

Accelerator Mass Spectrometry
For analysis of the abundance of a cosmogenic nuclide by accelerator mass spectrometry (AMS), the element is first chemically extracted from the sample (for example, a rock, rain water sample or a meteorite), then loaded into a copper holder and inserted into the mass spectrometer through a vacuum lock.

An ion source produces a beam of ions (atoms that carry an electrical charge), usually of cesium (Cs+) ions, by heating a few milligrams of solid Cs material in a hot ioniser. The Cs ions are accelerated and focused to a small spot on the sample. Negative secondary ions sputtered from the surface of the sample are extracted and sent down the evacuated beam line towards the first magnet. At this point the beam is typically about 10 microamps which corresponds to 1013 ions per second (mostly consisting of the stable isotopes).

The injector magnet bends the negative ion beam by 90° to select the mass of interest, a radioisotope of the element inserted in the sample holder, and reject the much more intense neighbouring stable isotopes. Vacuum pumps remove all the air from the beamline so the beam particles have a free path. There are still lots of molecules and isobars (isotopes of neighboring elements having the same mass) that must be removed by more magnets after the accelerator.

The tandem accelerator consists of two accelerating gaps separated by a large positive voltage inside a large pressure vessel that contains CO2 and N2 insulating gas at a pressure of over 10 atmospheres. The bridge holds two long vacuum tubes with many electrically insulating glass sections. The center of the accelerator, the terminal, is charged to a voltage of up to 10 million volts by two rotating chains. Negative ions traveling down the beam tube are attracted (accelerated) towards the positive terminal. At the terminal they pass through an electron stripper, either a gas or a thin carbon foil, and emerge as positive ions. These are then repelled from the positive terminal, accelerating again to ground potential at the far end. The name tandem accelerator comes from this dual acceleration concept. The final velocity is a few percent of the speed of light or about 50 million miles per hour.

The analyzing and switching magnets select the mass of the radionuclide of interest, further reducing the intensity of neighbouring stable isotopes. In addition, they eliminate molecules completely by selecting only the highly charged ions that are produced in the terminal stripper. Highly charged molecules are unstable since they are missing the electrons that bind the atoms together. Isotope ratios are measured by alternately selecting the stable and radioisotopes with the injector and analyzing magnets.

The electrostatic analyzer is a pair of metal plates at high voltage that deflects the beam by about 20°. This selects particles based on their energy and thus removes potentially interfering ions from the accelerator.

The gas ionization detector counts ions one at a time as they come down the beamline. The ions are slowed down and come to rest in propane gas. As they stop, electrons are knocked off the gas atoms. These electrons are collected on metal plates, amplified, and read into the computer. For each atom, the computer determines the rate of energy loss and from that deduces the nuclear charge (element atomic number) to distinguish interfering isobars.